Prior to human consumption, vegetable oil undergoes processing which generally includes bleaching, deodorization and the removal of unwanted particulate material. The unwanted particulate material includes wax which shall mean for the purposes herein high melting glycercides such as saturated glycerides having 16 to 18 carbons.
Typically vegetable oil is extracted from seed, refined and bleached. After bleaching, the hot oil, usually about 120.degree. F., may be transferred to a "winterization" unit. Winterization is the process by which higher-melting glycerides are crystallized for removal from the oil. Some vegetable oils require winterization and the removal of higher melting glycerides to avoid problems in the use of the oils at lower temperatures and in later processing. Other vegetable oils do not require winterization to remain processable at lower temperatures. A detailed discussion of winterization of vegetable oil is provided in Industial Oil and Fat Products by Alton E. Bailey (Interscience Publishers, 1951) at pages 868 through 872 which discussion is incorporated herein by reference.
The waxes are separated from the oils by employing crystallization techniques. After the wax is crystallized, the wax is separated from the oils in filter presses using pre-coated plates of diatomaceous earth.
Prior to this invention, it has been generally thought that during winterization for the formation of wax crystals in the oils, the temperature of the oils are lowered at a slow cooling rate to allow crystallization so that a true solid liquid separation process can be effected. It also has been known to add a small amount of seed material to allow the waxes to nucleate and grow. About 3% to about 5%, based upon the weight of the oil, of diatomaceous earth has been added as a seed material to the chilled oil and mixed. It has been generally believed, however, that the oil should be cooled slowly and carefully to avoid difficulty in the filtration process. This slow cooling has not been necessarily energy efficient particularly if heating is required to control the cooling rate. Further, the slow cooling of the oil would slow the production of winterized refined oil.
Generally in the filtration process, it has been known to pass the oil/wax particulate mixture perpendicular to the pre-coated plate and frame filter with dead-end pressure filtration to form a cake. The cake then serves as a filter to capture the particles, but allows the oil to permeate through the cake. Prior to filtration, the typical inlet concentration of wax in the case of, for example, sunflower oil, is between about 300 ppm to about 1000 ppm. The filtered oil for commercial or industrial use should pass a three day chill test and have a particulate concentration of less than about 50 ppm.
After operation of the plate and frame filter press, the diatomaceous earth is soaked with oil and is a waste product which requires disposal. Analysis of samples of this diatomaceous earth material indicates that the material comprises about 50 to about 65 weight percent oil. In one plant, about 544 tons of diatomaceous earth is used annually. This means that just from one plant, there is a loss of hundreds of tons of oil annually from the filtration of wax from oil. Therefore, it is desirable to develop a process for removing waxes and other particulate materials from oils without this substantial loss of oil, not only to reduce costs arising from lost oil, but also to reduce costs and environmental problems arising from the necessity of the disposal of the diatomaceous earth soaked with oil.
Another problem with dead end filtration with diatomaceous earth is that once a cake is formed, the pre-coat of diatomaceous earth adheres very strongly to the filter. Mechanical rakes are necessary to clean the surface of the filter. The mechanical raking is labor intensive and is therefore expensive. Thus, it is highly desirable to develop a process that would not require such a substantial effort for cleaning the filter presses.
Yet another problem with the dead end plate and frame filtration process is that the wax particles and the oil travel in the same direction and the flow of the feed to the filter is perpendicular to the surface of the filter. Therefore over a period of time, particulate wax material builds up as a cake and the cake resistance is greater than the filter resistance. Therefore, the filtration declines steadily as a function of time. It is highly desirable to develop a process in which the filtration operation is continuous and does not readily decline as a function of time.
It is an object of this invention to provide a rapid energy efficient continuous process for treating vegetable oils, which process is effective without a slow cooling of the oil, but which is economic and produces a vegetable oil that is not turbid at lower temperatures.
It is also an object of this invention to provide a process which eliminates the need for filter aids, such as diatomaceous earth, in the separation of particulate materials from vegetable oils.
It is another object of this invention to reduce or eliminate the generation of wax permeated with diatomaceous earth.
Still further objects and advantages of the invention will be found by reference to the following description.